Method for producing all solid state battery

ABSTRACT

A method for producing an all solid state battery using a precipitation-dissolution reaction of metallic Li as a reaction of an anode, includes a preparation step, a liquid composition preparation step, a coating layer formation step, and a separator formation step. The preparation step includes preparing a sulfide solid electrolyte represented by Li7-aPS6-aXa (X is at least one of Cl, Br, and I, and a satisfies 0≤a≤2), the liquid composition preparation step includes dissolving the sulfide solid electrolyte in an alcohol-based solvent to prepare a liquid composition, the coating layer formation step includes applying the liquid composition to an anode current collector to form a coating layer, the separator formation step includes forming a separator by volatilizing the alcohol-based solvent from the coating layer by drying, and the ratio of the sulfide solid electrolyte contained in the liquid composition is 10% by weight to 30% by weight.

TECHNICAL FIELD

This application is a continuation of U.S. application Ser. No.17/476,156, filed Sep. 15, 2021 (allowed), which claims the benefit ofJapanese Patent Application Nos. 2020-158403, filed Sep. 23, 2020 and2021-009292 filed Jan. 25, 2021. The present disclosure relates to amethod for producing an all solid state battery.

BACKGROUND

An all solid state battery is a battery having separators (solidelectrolyte layers) between the cathode layer and the anode layer, andhas advantages of simplifying the safety device as compared with aliquid battery having an electrolyte containing combustible organicsolvents.

For example, Patent Literature 1 discloses a lithium-solid secondarybattery in which a green compact of sulfide solid electrolyte is formedon an anode current collector. In this battery, metallic Li isprecipitated and dissolved between the anode current collector and thecompact as a reaction of an anode.

PRIOR ART DOCUMENTS

[Patent Document 1] JP-A-2016-012495

SUMMARY Problem to be Solved

In a battery utilizing precipitation-dissolution reactions of metallicLi, it is essential to suppress the generation of short circuits due todendrites. Here, if the filling ratio of the separator separating thecathode and anode current collector is low, dendrites tend to grow alongthe grain boundaries and voids are created between the particles in theseparator, so that occurrence of short circuits may not be sufficientlysuppressed.

The present disclosure has been made in view of the above circumstances,and a main object of the present disclosure is to provide a method formanufacturing an all solid state battery having separators having a highpacking ratio.

Means for Solving the Problem

In order to solve the above problem, in the present disclosure, there isprovided a method for producing an all solid state battery. A method forproducing an all solid state battery using a precipitation-dissolutionreaction of metallic Li as a reaction of an anode, comprises apreparation step, a liquid composition preparation step, a coating layerformation step, and a separator formation step, wherein the preparationstep is a step of preparing a sulfide solid electrolyte represented byLi_(7-a)PS_(6-a)X_(a) (X is at least one of Cl, Br, and I, and asatisfies 0≤a≤2), the liquid composition preparation step is a step ofdissolving the sulfide solid electrolyte in an alcohol-based solvent toprepare a liquid composition, the coating layer formation step is a stepof applying the liquid composition to an anode current collector to forma coating layer, the separator formation step is a step of forming aseparator by volatilizing the alcohol-based solvent from the coatinglayer by drying, and the ratio of the sulfide solid electrolytecontained in the liquid composition is 10% by weight or more and 30% byweight or less.

According to the present disclosure, since a predetermined liquidcomposition is used, an all solid state battery provided with aseparator having a high filling ratio can be manufactured.

In the above disclosure, the above sulfide solid electrolyte may be asulfide glass.

In the above disclosure, the variable “a” may be 2.

In the above disclosure, the variable “X” may be at least I.

In the above disclosure, the above alcohol-based solvent may containethanol.

In the above disclosure, in the separator formation step, a dryingpressure may be a normal pressure, a drying temperature may be(T_(B)+10°) C or less when a boiling point of the alcohol-based solventis regarded as T_(B)° C., and a drying time may be a time until aresidual solvent of the alcohol-based solvent becomes 0.53% by weight orless.

Effect of the Invention

In the present disclosure, it is advantageous to be able to produce anall solid state battery with high-fill separators.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram illustrating an exemplary method for producingan all solid state battery in the present disclosure.

FIGS. 2A and 2B are schematic cross-sectional view illustrating anexemplary all solid state battery according to the present disclosure.

FIGS. 3A and 3B are graphs showing the relationship between the solidfraction and the filling ratio and the capacity retention ratio in theexamples and comparative examples.

FIG. 4 is a graph showing the relationship between the dryingtemperature and the ion conductivity in the examples.

FIG. 5 is a graph showing the relationship among the drying temperature,the solvent residue, and the filling ratio.

DESCRIPTION

A method for producing an all solid state battery in the presentdisclosure will be described in detail.

FIG. 1 is a flowchart showing an exemplary method for manufacturing anall solid state battery according to the present disclosure. In FIG. 1 ,first, a sulfide solid electrolyte represented by Li_(7-a)PS_(6-a)X_(a)is prepared (X is at least one of Cl, Br, and I, and a satisfies 0≤a≤2)(preparing step). Next, a liquid composition is prepared by dissolvingsulfide solid electrolyte in an alcohol-based solvent (liquidcomposition preparation step). Next, the above liquid composition isapplied to an anode current collector to form a coating layer (coatinglayer formation step). Then, a separator is formed by volatilizing analcohol-based solvent from the coating layer by drying (separatorformation step). In addition, the ratio of sulfide solid electrolytecontained in the liquid composition is within a predetermined range.

According to the present disclosure, since a predetermined liquidcomposition is used, an all solid state battery provided with aseparator having a high filling ratio can be manufactured.

As in Patent Literature 1 described above, in an all solid state batteryutilizing a precipitation-dissolution reaction of metallic Li, it isknown to form a green compact of sulfide solid electrolyte on an anodecurrent collector as a separator. Since such compacts are formed bycompacting particulate solid electrolyte, grain boundaries and gapsbetween particles are generated, and there are limitations in raisingthe packing rate. Further, in order to improve the filling rate of thegreen compact, increasing the press pressure and press temperature orperforming sintering are conceivable, but mass productivity is lowered.On the other hand, in the present disclosure, a predetermined liquidcomposition containing sulfide solid electrolyte in a predeterminedrange is used. In such a liquid composition, the proportion ofundissolved sulfide solid electrolyte is small. Therefore, in theseparator obtained by coating and drying this liquid composition, thereis no grain boundary of sulfide solid electrolyte and a higher fillingratio is obtained. Therefore, in all solid state battery having suchseparators, the number of voids in which dendrites can grow is small, sothat the generation of short-circuiting can be further suppressed. Inaddition, when the packing ratio of the separator is high, it ispossible to suppress invasive precipitation of the metal Li in the voidsin the separator, so that it is possible to satisfactorily precipitateand dissolve the metal Li on the current collector. Consequently,battery characteristics such as cyclic characteristics are also good.

1. Preparation Step

The preparation step in the present disclosure is a step of preparing asulfide solid electrolyte represented by Li_(7-a)PS_(6-a)X_(a), where“X” is at least one of Cl, Br, and I, and “a” satisfies 0≤a≤2.

In Li_(7-a)PS_(6-a)X_(a), “X” is at least one of Cl, Br, and I. “X” maybe only Cl, may be only Br, and may be only I. Further, “X” may be 2 ormore of Cl, Br and I. In particular, “X” is preferably at least I.

Further, in Li_(7-a)PS_(6-a)X_(a), “a” satisfies 0≤a≤2. “a” may be 0 orgreater than 0. In the latter case, “a” may be 0.5 or more, and may be1.0 or more. On the other hand, “a” is 2 or less, and may be 1.5 orless. When “a” is 2, the filling rate is likely to be improved.

Sulfide solid electrolyte in this disclosure may be a sulfide glass, maybe a glass ceramic, and may be a crystalline material. When the sulfidesolid electrolyte is sulfide glass, it is easily dissolved in analcohol-based solvent to be described later.

In the preparation step, the above-described sulfide solid electrolytemay be purchased and prepared, and may be synthesized and prepared. Whensulfide glasses are synthesized as sulfide solid electrolyte, they canbe obtained, for example, by subjecting the raw material composition toan amorphization treatment. The raw material composition contains Li₂S,P₂S₅ and LiX. “X” is as described above. Examples of LiX include LiI,LiCl and LiBr. Incidentally, it is3.5((1−α)Li₂S·αLiX)·0.5P₂S₅→Li_(7-3α)PS_(6-3.5α)X_(3.5α); thus, whena=3.5α, it is Li_(7-a)PS_(6-a)X_(a).

Examples of the amorphization process include mechanical milling such asa ball mill, a vibration mill, a turbo mill, mechanofusion, and a diskmill. Of these, a ball mill is preferred, and particularly a planetaryball mill is preferred. The mechanical milling may be dry mechanicalmilling or wet mechanical milling.

When glass ceramics are synthesized, it can be obtained by heat-treatingthe above-mentioned sulfide glass. In addition, when a crystallinematerial is synthesized, it can be obtained, for example, by subjectingthe raw material composition to a solid phase reaction treatment. Inaddition, when sulfide solid electrolyte in the present disclosure has acrystal phase, that crystal phase is preferably an argyrodite type.

It is preferable that the sulfide solid electrolyte has high ionicconductivity. Ion conductivity at 25° C. is, for example, 10⁻⁴ S/cm ormore and may be 10⁻³ S/cm or more. The shape of sulfide solidelectrolyte is not particularly limited, and examples thereof includeparticulate. The average particle size D₅₀ of sulfide solid electrolyteis, for example, 0.1 μm or more and 50 μm or less. The average particlesize can be determined, for example, from the results of particle sizedistribution measurement by laser diffraction scattering method.

2. Liquid Composition Preparation Step

The liquid composition preparation step in the present disclosure is astep of dissolving the above sulfide solid electrolyte in analcohol-based solvent to prepare a liquid composition. In addition, theratio of sulfide solid electrolyte contained in the liquid compositionis 10% by weight or more and 30% by weight or less.

Alcohol-based solvents have a high solubility in sulfide solidelectrolyte (sulfide solid electrolyte represented byLi_(7-a)PS_(6-a)X_(a)) and can dissolve a large amount of sulfide solidelectrolyte. Further, since the alcohol-based solvent has a relativelylow boiling point, it is easy to remove the alcohol-based solvent.Further, the alcohol-based solvents hardly cause degradation of theabove sulfide solid electrolyte due to dissolution.

The type of the alcohol-based solvent is not particularly limited, andexamples thereof include a primary alcohol. The number of carbon atomsof the alcohol-based solvent is, for example, 1 or more and 6 or less,and may be 1 or more and 3 or less. Specific examples of thealcohol-based solvent include ethanol, methanol and propanol, forexample. The alcohol-based solvent may be used alone, or 2 or more ofthem may be used in combination. In particular, the alcohol-basedsolvent in the present disclosure is preferably a solvent containingethanol as a main component. The ratio of ethanol to the entirealcohol-based solvent is, for example, 50% by weight or more, and may be70% by weight or more, and may be 90% by weight or more.

The ratio of the above sulfide solid electrolyte contained in the aboveliquid composition is usually 10% by weight or more, and may be 15% byweight or more. On the other hand, the ratio of the above sulfide solidelectrolyte contained in the above liquid composition is usually 30% byweight or less, and may be 25% by weight or less, and may be 20% byweight or less. If the ratio of the above sulfide solid electrolyte istoo low or too high, a satisfactory filling ratio may not be obtained insome cases.

In addition, the liquid composition may contain only the above sulfidesolid electrolyte as a solid content, and may contain other materials inaddition to the above sulfide solid electrolyte. The ratio of the abovesulfide solid electrolyte to the total solid content is, for example,50% by weight or more, and may be 70% by weight or more, and may be 90%by weight or more. On the other hand, the above ratio is, for example,99% by weight or less, and may be 95% by weight or less. Examples ofother materials include solid electrolyte and binders other than sulfidesolid electrolyte in the present disclosure described above.

Sulfide solid electrolyte contained in the liquid composition may becompletely dissolved in an alcohol-based solvent. On the other hand, apart of the sulfide solid electrolyte contained in the liquidcomposition may not be dissolved in an alcohol-based solvent. In thisinstance, a portion of sulfide solid electrolyte is present insuspension in alcohol-based solvents.

The liquid composition is prepared by mixing the above sulfide solidelectrolyte with the above alcohol-based solvents. The mixing methodincludes, for example, an ultrasonic homogenizer, a shaker, a thin filmswirl mixer, a dissolver, a homomixer, a kneader, a roll mill, a sandmill, an attritor, a ball mill, a vibrator mill, and a high-speedimpeller mill.

3. Coating Layer Formation Step

The coating layer formation step in the present disclosure is a step offorming a coating layer by applying the above liquid composition to ananode current collector.

Since the liquid composition is the same as that described above,description thereof will be omitted here. The anode current collectormay be similar to the anode current collector commonly used in all solidstate battery. Examples of the material of the anode current collectorinclude SUS, copper, nickel, and carbon. Methods of coating the liquidcomposition include, for example, a doctor blade method, a die coatmethod, a gravure coat method, a spray coating method, a static coatingmethod, and a bar coating method. The thickness of the coating layer isnot particularly limited, and can be appropriately adjusted so as toobtain a desired thickness of the separators.

4. Separator Formation Step

The separator formation step in the present disclosure is a step offorming a separator by volatilizing the above-mentioned alcohol-basedsolvent from the above coating layer by drying.

In the separator formation step, the above alcohol-based solvents arevolatilized from the above coating layer by drying. Thus, thealcohol-based solvent can be removed from the coating layer, and sulfidesolid electrolyte can be redeposited. The drying method is notparticularly limited as long as the above-mentioned alcohol-basedsolvent can be volatilized, and examples thereof include general methodssuch as warm air and hot air drying, infrared drying, vacuum drying, anddielectric heating drying.

A drying pressure (pressure during drying) may be a normal pressure andmay be a reduced pressure, but the former is preferred. If it is anormal pressure, occurrence of bumping of the alcohol-based solventduring drying can be inhibited, and generation of voids inside theseparator can be inhibited. As a result, a separator with high fillingratio can be obtained. The normal pressure refers to an atmosphericpressure, and it is typically 1 atm, but the pressure of 0.5 atm or moreand 1.5 atm or less is acceptable. On the other hand, in the lattercase, the drying pressure is, for example, 0.01 atm or less.

Further, examples of the dry atmosphere include an inert gas atmospheresuch as an Ar gas atmosphere and a nitrogen gas atmosphere, an airatmosphere, and a vacuum. A gas such as an inert gas may be allowed toflow during drying. The drying temperature is not particularly limited,but is preferably a temperature at which sulfide solid electrolyte doesnot deteriorate. When the boiling point of the alcohol-based solvent isregarded as T_(B)° C., the drying temperature is, for example,preferably (T_(B)+10°) C or less, and more preferably T_(B)° C. or less.If the drying temperature is too high, bumping of the alcohol-basedsolvent during drying would easily occur, and thus the flow rate whenthe solvent vapor escapes from the separator would increase to easilygenerate voids and defects. As a result, the filling ratio may easilydecrease. On the other hand, by setting the drying temperature to notmore than the vicinity of the boiling point of the alcohol-basedsolvent, the bumping of the alcohol-based solvent during drying wouldnot easily occur. Furthermore, the drying temperature is, for example,preferably (T_(B)−30°) C or more, and more preferably (T_(B)−20°) C ormore. If the drying temperature is too low, the alcohol-based solventwould easily remain after drying, and there may be a case where the Liion conductivity of the separator decreases. Also, if the dryingtemperature is too low, drying would take time and there may be a casewhere productivity drops.

In the separator formation step, all of the alcohol-based solvents maybe substantially volatilized and removed from the coating layer, and apart thereof may be volatilized and removed, but the former ispreferred. A drying time is not particularly limited, but may be a timeuntil the residual solvent of the alcohol-based solvent becomes, forexample, 5.1% by weight or less, may be 2.1% by weight or less, and maybe 0.53% by weight or less. The less the residual solvent becomes, themore the Li ion conductivity of the separator easily improves. Thedrying time is, for example, 10 minutes or more, and may be 30 minutesor more. Meanwhile, the drying time is, for example, 10 hours or less,may be 5 hours or less, and may be 2 hours or less.

Sulfide solid electrolyte contained in the separator after drying may bea sulfide glass, a glass ceramic, or a crystalline material. If theabove sulfide solid electrolyte has a crystal phase, that crystal phaseis preferably argyrodite type. The thickness of the separator is notparticularly limited, but is, for example, 15 μm or less, and may be 10μm or less. On the other hand, the thickness of the separator is, forexample, 0.5 μm or more, and may be 1 μm or more, and may be 5 μm ormore. If the thickness of the separators is too large, the reversibilityof the precipitation-dissolution reactions of metallic Li becomes low,and battery performance such as cycling characteristics may bedeteriorated. On the other hand, if the thickness of the separator istoo small, the occurrence of a short circuit may not be effectivelysuppressed in some cases. The thickness of the separator can beadjusted, for example, by changing the coating amount of the liquidcomposition.

Also, the filling ratio of the separator is, for example, 94% or more,may be 96% or more, and may be 98% or more. Further, the Li ionconductivity of the separator at 25° C. is, for example, 0.1 mS/cm ormore, may be 0.2 mS/cm or more, and may be 0.3 mS/cm or more.

5. Other Steps

In the present disclosure, a separator disposed between a cathode layerand an anode layer may be formed by performing only the separatorformation step described above, and may be formed by further performingother steps. In the latter case, the “separator formation step”described above can be referred to as a “first separator formationstep”. The first separator is formed by the first separator formationstep. On the other hand, another step may be referred to as, forexample, a “second separator formation step”, and a second separator isformed by the second separator formation step. The second separatorformation step is, for example, a step of forming a green compact of theabove sulfide solid electrolyte. The second separator is preferablydisposed between the first separator and cathode layers. By forming thesecond separator in addition to the first separator, the thickness ofthe entire separator can be easily improved as compared with the casewhere only the first separator is formed. The thickness of the secondseparator is, for example, 0.1 μm or more and 1000 μm or less.

The present disclosure may further include a cathode formation step offorming a cathode having a cathode active material layer and a cathodecurrent collector, a stack forming step of forming a laminate having aanode current collector, a separator, a cathode active material layer,and a cathode current collector in this order, and a pressing step ofpressing the laminate. In addition, in the stack formation step, a greencompact (second separator) of the above sulfide solid electrolyte may bedisposed between the separator and the above cathode active materiallayer. The cathode formation step, the laminating step, and the pressingstep may be conventionally known methods.

6. All Solid State Battery

FIGS. 2A and 2B are schematic cross-sectional view illustrating anexample of an all solid state battery in the present disclosure. Allsolid state battery 10 shown in FIG. 2A includes an anode currentcollector 5, a first separator 2, a second separator 3, a cathode activematerial layer 1, and a cathode current collector layer 4 in this order.The first separator 2 is a separator formed in the above-describedseparator formation step. All solid state battery 10 is a battery usinga precipitation-dissolution reaction of metallic Li as an anodereaction. Therefore, as shown in FIG. 2B, in an all solid state battery10, anode active material layers 6, which are deposited Li, are formedbetween anode current collector 5 and the first separators 2 by charge.

Materials of the cathode current collector, cathode active materiallayers and anode active material layers may be conventionally known.Since the anode current collector is the same as that described above,description thereof will be omitted here.

The all solid state battery may be a primary battery or a secondarybattery, and among them, a secondary battery is preferable. This isbecause it may be charged and discharged repeatedly, and it is useful,for example, as an in-vehicle battery. Battery shapes may include, forexample, coin-shaped, laminated, cylindrical and square-shaped, and thelike.

Note that the present disclosure is not limited to the above embodiment.The above-mentioned embodiments are illustrative, and any one havingsubstantially the same configuration as the technical idea described inthe claims in the present disclosure and exhibiting the same operationand effect is included in the technical scope in the present disclosure.

EXAMPLES Example 1 (Form of Separator)

Sulfide glass (Li₆PS₅Cl₁) was synthesized by weighing Li₂S, P₂S₅ andLiCl as starting materials and mechanically ball milling them. 400 mg ofthis sulfide glass was charged into a glass bottle, to which ethanol wasadded dropwise so that the solid content was 10 wt % and stirred for 3minutes. Thus, a yellow transparent solution (liquid composition) wasobtained. This liquid composition was applied onto a Cu foil (anodecurrent collector) using a 100 μm-gap SUS blade to form a coating layer.This coating layer was dried in a glove box at 60° C. for 1 hours.Thereafter, it was dried in vacuo at 120° C. for 10 minutes. As aresult, a member having an anode current collector and a separator thinfilm (11.7 micrometers thick) was obtained.

(Forming a Cathode)

As a cathode material, cathode active material(LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂), the above sulfide glass, and anelectrical material (VGCF-H: Showa Denko K.K.) were prepared. These wereweighed to a total 2 g in a volume ratio of 78:19:3 and mixed. To thismixture, 1200 mg of butyl butyrate and 20 mg of PVDF binder were addedand disintegrated in an ultrasonic homogenizer. Thus, a cathode slurrywas produced. Cathode slurries were coated onto an Al-foil (cathodecurrent collector) using a 300-μm-gap SUS blade. Thereafter, it wasdried at 100° C. for 1 hour. This gave a cathode with cathode layers anda cathode current collector.

(Production of all Solid State Battery)

100 mg of the above sulfide glasses were weighed and put intocylindrical cylinders with a diameter of 11.28 mm and pressure-molded at1 ton. Thus, electrolyte pellets were prepared. A cathode layer wasplaced on the upper surface of the pellet (second separator), and aseparator (first separator) was pressed at 6 tons on the lower surface.Thus, an all solid state battery was produced. The produced all solidstate battery was restrained by 3 MPa. When the above-mentioned sulfideglasses, active materials, and conductive materials were handled, theoperation was carried out in a glove box in which the dew point wasadjusted to −70° C. or less in an Ar-gas atmosphere.

Example 2 to 7

Starting materials and their percentages were adjusted so that sulfideglass (Li_(7-a)PS_(6-a)X_(a)) had the composition shown in Table 1. Inaddition, liquid compositions were prepared so that the solid fractionbecame a numerical value shown in Table 1. Liquid composition was coatedon the anode current collector so that the thickness of the separator(first separator) became the value shown in Table 1. Except for these,an all solid state battery was produced in the same manner as in Example1.

Comparative Example 1

An all solid state battery was produced in the same manner as in Example1 except that the separators were formed as described below. First,sulfide glasses were synthesized in the same manner as in Example 1.This sulfide glass and ABR binder were weighed to a total 1 g in avolume ratio of 98:2 and mixed. To this, 2000 mg of heptane was addedand crushed by an ultrasonic homogenizer to prepare a slurry. The slurrywas coated on a Cu foil with a 100 m gap SUS blade and dried at 100° C.for 1 hour. By this, the separator thin film of 15 m thickness wasobtained.

Comparative Example 2

The solid fraction of the liquid composition and the thickness of theseparator were changed as shown in Table 1. Except for this, an allsolid state battery was produced in the same manner as in EXAMPLE 1.

Comparative Example 3 and 4

The solid fraction of the liquid composition and the thickness of theseparator were changed as shown in Table 1. Except for this, an allsolid state battery was produced in the same manner as in Example 7.

[Evaluation] (Filling Rate)

The filling ratio of the separators produced in Examples 1 to 7 andComparative Examples 1 to 4 was determined by the following method.First, weighed separator was charged into a cylindrical cylinder havingφ11.28 mm, and restrained at 3 MPa. The thickness of the separator atthis time was measured by a film thickness meter. Next, the apparentdensity of the separator was calculated from the area, thickness andmass of the produced separator (apparent density of theseparator=mass/(thickness×area)). The true density of the separator wasthen calculated from the true density and content of the components ofthe separator (sulfide glasses). (True density ofseparators=mass/sulfide glass content/True density of sulfide glass).The ratio of the apparent density to the true density was defined as thefilling ratio (%). The results are shown in Table 1 and FIG. 3A.

(Capacity Maintenance Ratio)

The all solid state battery prepared in Examples 1 to 7 and ComparativeExamples 1 to 4 were connected to a charge/discharge tester and cycledat 2.8-4.3 V and 0.1 C while being maintained at 25° C. The number ofcycles was 10. The capacity retention rate at the tenth cycle wascalculated. The results are shown in Table 1 and FIG. 3B.

TABLE 1 Capacity Solid Filling retention Thickness fractionLi_(7-a)PS_(6-a)X_(a) ratio ratio (μm) ( wt %) X a (%) ( % ) Comp.Example 1 15.0 — Cl 1 87.3 46.2 Comp. Example 2 11.2  5 Cl 1 89.4 55.7Example 1 11.7 10 Cl 1 93.7 74.6 Example 2 10.4 10 I 1 94.4 77.1 Example3 12.5 10 I 2 96.3 82.2 Example 4 11.3 20 I 1 97.7 88.1 Example 5 11.220 I 2 99.1 95.1 Example 6 11.7 30 I 1 94.1 75.6 Example 7 10.4 30 I 295.7 83.3 Comp. Example 3 12.5 40 I 2 85.9 67.9 Comp. Example 4 11.3 50I 2 80.6 57.3

As shown in Table 1 and FIGS. 3A and 3B, the filling ratio and thecapacity retention ratio of Examples 1 to 7 were improved as comparedwith Comparative Examples 1 to 4. In particular, in Examples 4 and 5having a solid fraction of 20 wt %, the filling ratio and the capacityretention ratio were particularly good. This is presumably a saturatedconcentration of sulfide solid electrolyte in the vicinity of a solidfraction of 20 wt %. On the other hand, in Comparative Example 2 inwhich the solid fraction was 5 wt %, the filling ratio and the capacityretention ratio were low. This is considered to be due to the fact thatthe amount of the alcohol-based solvent increased relative to eachother, so that voids were generated when the alcohol-based solvent wasremoved in drying. In addition, in Comparative Examples 3 and 4 in whichthe solid fraction exceeded 30 wt %, the filling ratio was remarkablylowered. This is considered to be because the amount of sulfide glasswhich was not dissolved in the liquid composition was large, and theparticles of the glass were incorporated into the separator thin film,thereby causing grain boundaries and voids of the particles themselves.Further, it was confirmed that the higher the halogen content in sulfideglasses, the higher the filling ratio and the maintaining ratio, and itwas confirmed that the filling ratio and the maintaining ratio areimproved in the same manner when the halogen species is changed from Clto I. This is considered to be because the densification of theseparators was further promoted by increasing the true density ofsulfide glass and decreasing the Young's modulus of the glass by addingI.

Example 8

Sulfide glass (Li₆PS₅Cl₁) was synthesized by weighing Li₂S, P₂S₅ andLiCl as starting materials and mechanically ball milling them. 400 mg ofthis sulfide glass was charged into a glass bottle, to which ethanol wasadded dropwise so that the solid content was 20 wt % and stirred for 3minutes. Thus, a yellow transparent solution (liquid composition) wasobtained. This liquid composition was applied onto a Cu foil (anodecurrent collector) using a 100 m-gap SUS blade to form a coating layer.This coating layer was dried in a glove box at 60° C. for 5 minutes.Thereafter, it was dried in vacuo (0.01 atm) at 120° C. for 10 minutes.As a result, a separator thin film (10 micrometers thick) on an anodecurrent collector was obtained.

Example 9

A separator thin film was obtained in the same manner as in Example 8except that the conditions of drying the coating layer was changed to 2hours at 25° C. in a glove box having Ar flow atmosphere (normalpressure).

Examples 10 to 14

A separator thin film was obtained in the same manner as in Example 9except that the drying temperature was changed to those shown in Table1.

[Evaluation] (Filling Ratio and Li Ion Conductivity)

The filling ratio of the separators produced in Examples 8 to 14 wasdetermined by the following method. First, weighed separator was chargedinto a cylindrical cylinder having φ11.28 mm, and restrained at 3 MPa.The thickness of the separator at this time was measured by a filmthickness meter. Next, the filling ratio was calculated from the area,thickness and mass of the produced separator. The results are shown inTable 2 and FIG. 4 . Next, the restrained separator was connected to apotentiostat (VMP from Biologic) having a Frequency Response Analyzer(FRA), and impedance was measured while maintaining 25° C. so as toobtain Cole-Cole plot. The point where the obtained electron blockingspectrum crossed the real axis was regarded as an ion conductionresistance, and the ion conductivity was calculated from that resistanceand the thickness of the separator. The results are shown in Table 2 andFIG. 5 .

(Solvent Residue)

The solvent residue amount of the separator produced in examples 8 to 14was determined from the following method. First, the separator wasscraped by a spatula, so as to obtain 10 mg to 20 mg powder. Theobtained powder was placed in an Al pan, the surface thereof wassmoothed, and simultaneous thermogravimetry/differential thermalanalysis (TG-DTA) was conducted thereto. The temperature rising speedwas 10° C./min, the measurement temperature was 25° C. to 150° C., andthe atmosphere was Ar flow. In the TG curve, weight due to theevaporation of ethanol that appears in the vicinity of 80° C. wascalculated and determined as the solvent residue after drying. Theresults are shown in Table 2 and FIG. 4 .

TABLE 2 Drying Drying temper- Solvent Filling pressure ature Dryingresidue ratio Li ion (atm) (° C.) time (wt %) ( % ) (mS/cm) Example 8 1 60 5 min  0.47 93.7 0.12    0.01 120 10 min Example 9 1  25 2 hours 5.198.4 0.018 Example 10 1  40 2 hours 2.1 98.8 0.11  Example 11 1  60 2hours  0.51 98.7 0.40  Example 12 1  80 2 hours  0.53 98.9 0.38  Example13 1 100 2 hours  0.44 94.1 0.29  Example 14 1 120 2 hours  0.41 92.40.24 

As shown in Table 2, in example 8, the solvent residue after dryingreached at 0.47 wt %, and thus it is presumed that the solvent wasalmost completely (substantially) removed. On the other hand, comparedto example 8, the filling ratio of examples 9 to 12 was extremely highwhich was 98% or more. It is presumably because vacuum drying caused thesolvent to vaporize rapidly, and the solvent escaped while creatingvoids and defects in the separator in example 8, and the solventvaporized slowly due to atmospheric drying, and the solvent escapedwithout creating voids and defects in the separator in examples 9 to 12.Further, particularly in examples 11 and 12, the filling ratio reactedat about 99%, and the Li ion conductivity also almost matched the valueshown in the document (Journal of Power Sources Volume 389, 15 Jun.2018, Pages 140-147) where synthesis was conducted by a solid phasemethod.

Furthermore, the solvent residue in examples 9 and 10 was significantlymore than that of examples 11 to 14. It is presumably because the dryingtemperature was low and the removement of solvent did not sufficientlyproceed. As shown in FIG. 4 and FIG. 5 , when the drying temperature waslow, although the filling ratio was high, the solvent residue was moreand the Li ion conductivity was low. On the other hand, in examples 11to 14, the solvent residue dropped to the same as example 8 (vacuumdrying), and the solvent was almost completely removed. However, asshown in FIG. 4 and FIG. 5 , when the drying temperature was 100° C. ormore, decreases in filling ratio and Li ion conductivity were confirmed.This is presumably because bumping of ethanol (boiling point: 78° C.)became dominant when the drying temperature was 100° C. or more, and theflow rate when the solvent vapor escaped from the separator increased togenerate voids and defects. From these points, it was confirmed thatdrying at a normal pressure is effective for improving the fillingratio, and that the drying temperature on that occasion is preferablyaround the boiling point of the solvent.

DESCRIPTION OF SYMBOLS

-   -   1 . . . cathode active material layers    -   2 . . . first separator    -   3 . . . second separator    -   4 . . . cathode current collector    -   5 . . . anode current collector    -   6 . . . anode active material layers    -   10 . . . all solid state battery

What is claimed is:
 1. An all solid state battery utilizing aprecipitation-dissolution reaction of metallic Li as a reaction of ananode, obtained by a process comprising: a preparation step of preparinga sulfide solid electrolyte represented by Li_(7-a)PS_(6-a)X_(a) (X isat least one of Cl, Br, and I, and a satisfies 0≤a≤2), a liquidcomposition preparation step of dissolving the sulfide solid electrolytein an alcohol-based solvent to prepare a liquid composition, a coatinglayer formation step of applying the liquid composition to an anodecurrent collector to form a coating layer, and a separator formationstep of forming a separator by volatilizing the alcohol-based solventfrom the coating layer by drying, wherein the proportion of the sulfidesolid electrolyte contained in the liquid composition is 10% by weightor more and 30% by weight or less.